The present invention relates to a sealing device between two cavities at different pressures, for example, in an internal combustion engine fuel injector.
Known injectors normally comprise a hollow body carrying the nozzle; and a cavity at atmospheric pressure, in which slides a control rod for controlling the nozzle. The rod is controlled hydraulically by a metering valve comprising a valve body with a control chamber supplied with fuel under pressure.
The valve body of known injectors is substantially cylindrical, is housed inside a cylindrical seat in the hollow body, has an annular cavity for distributing fuel to the control chamber, and is therefore also subjected to high pressure and must be connected to the hollow body by a sealing device between the annular cavity and the cavity at atmospheric pressure.
For this purpose, provision is made, between the cylindrical wall of the valve body and the seat in the hollow body, for at least one annular seal, which normally rests on a shoulder of the seat, and is normally so sized that, when fitted to the valve body, it is stretched slightly to effectively seal the surface of the valve body. For technical reasons, the valve body has a 5 to 35 micron radial clearance with respect to the seat.
During operation of the injector, the high fuel pressure of around 1350 bar in the distribution cavity tends to force the seal inside the gap between the valve body and the seat, i.e. the seal is extruded inside the gap, thus resulting in the formation of extrusion rings and deterioration of the seal. As a result, the high-pressure fuel leaks increasingly through the extrusion rings, thus reducing the difference in pressure and generating heat due to leakage friction; which heat further impairs the resistance of the seal, which begins fraying and must therefore be changed frequently.